Lactic acid is widely used for foods, medical purposes, and chemical raw materials of cosmetics and the like. Furthermore, polylactic acid obtained using lactic acid is drawing attention as a biodegradable plastic which is finally decomposed into carbon dioxide and water by microorganisms and the like. Therefore, there is a need to produce lactic acid with high productivity at low cost.
As a process for production of lactic acid, a biological process for producing lactic acid by fermenting sugar with lactic acid bacteria is known. However, because lactic acid bacteria have poor acid resistance, in order to obtain high productivity in the aforementioned process, the lactic acid produced through fermentation needs to be changed into a lactate by being neutralized by an alkali. In the production process in which neutralization is performed by an alkali, a step of reverting the lactate to lactic acid is necessary. Accordingly, the production process becomes complicated, and the production costs increase.
As a process for obtaining lactic acid without performing neutralization by an alkali, there is a process using a transformant obtained by introducing a lactate dehydrogenase-encoding gene into yeast. For example, PTL 1 discloses a case where lactic acid can be produced with high productivity without performing a neutralization step with an alkali by conducting lactic acid fermentation by using a transformant which is obtained by incorporating a lactate dehydrogenase gene derived from mammals such as human beings into Schizosaccharomyces pombe and in which some of the genes in a group of pyruvate decarboxylase-encoding genes of the Schizosaccharomyces pombe host have been deleted or inactivated. Furthermore, PTL 2 discloses a case where L-lactic acid is obtained by culturing a transformant which is obtained by introducing an L-lactate dehydrogenase gene of Lactobacillus plantarum into Saccharomyces cerevisiae which substantially does not produce ethanol when cultured in a culture medium.